1. Field of the Invention
The present invention relates to an automatic feed system for a part to be resistance-welded and, more particularly, to an automatic feed system to be used when a T-shaped part such as a bolt is projection-welded to a work.
2. Description of the Prior Art
The projection welding of the bolt to the work is generally accomplished according to the following procedures First of all, the work (e.g., a steel sheet) is positioned with respect to the lower electrode of a resistance welder. This positioning is performed by aligning a welding preliminary hole formed in the work with a relief hole formed in the central portion of the lower electrode. When this positioning is ended, the bolt has its stem inserted into the welding hole of the work. Then, the bolt has its head pushed onto the work by the upper electrode of the welder coming down. In this state, a welding current is supplied between the upper and lower electrodes to effect the projection welding between the bolt and the work.
The projection welding thus far described is a manual one, but there is known a technology for feeding the bolt automatically to the welding preliminary hole of the work (as disclosed in Japanese Patent Publication No. 60-54832).
FIG. 9 shows a bolt feed system disclosed in the above-identified Publication. In this system, a guide pin 2 is axially movably fitted in the lower electrode 1 of a welder. The guide pin 2 is urged upwardly by a spring 3. The guide pin 2 can be inserted in a (welding preliminary ) hole 5 of a work 4 so that the work 5 is positioned with respect to the lower electrode 1. The bolt 6 to be welded is fed by a not-shown chute until it is chucked at the terminal end of the chute by chucking means 7.
The chucked bolt 6 is transferred to above an electrode push line, on which it is pushed by the upper electrode 8. In this case, the chuck 7 has its fingers 7a opened by the pushing action of the upper electrode 8 to push the bolt 6 downward. As a result, the bolt 6 is moved away from the chuck 7 to have its stem 6b inserted into the welding hole 5 of the work 4 and further into the lower electrode 1. When the upper electrode 8 is moved downward, the bolt 6 has its head projections 6a pushed onto the work 4. In this state, the welding current is supplied between the upper electrode 8 and the lower electrode 1 so that the bolt 6 is projection-welded at its projections 6a to the work 4.
In the system of the aforementioned system, however, the fingers 7a of the chuck 7 and the upper electrode slide into contact with each other each time the bolt 6 is pushed, so that the upper electrode 8 has its leading end seriously worn. As a result, the surface of the electrode for pushing the bolt has its flatness lost to raise a problem that the bolt is obliquely pushed. In order to avoid this problem, however, it is necessary to repair or replace the electrode frequently.
In this system, moreover, the chuck for chucking the bolt is influenced at each welding operation by the excess pressure of the electrode and the welding heat so that the durability of the chuck is dropped to make the chucking of the bolt instable. As a result, an mistaken insertion of the bolt is liable to occur and is difficult to detect. This raises another problem that stable welding qualities cannot be ensured.